sysdeps/libm-ieee754/s_cbrtf.c

   1 /* s_cbrtf.c -- float version of s_cbrt.c.
   2  * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
   3  */
   4
   5 /*
   6  * ====================================================
   7  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
   8  *
   9  * Developed at SunPro, a Sun Microsystems, Inc. business.
  10  * Permission to use, copy, modify, and distribute this
  11  * software is freely granted, provided that this notice
  12  * is preserved.
  13  * ====================================================
  14  */
  15
  16 #if defined(LIBM_SCCS) && !defined(lint)
  17 static char rcsid[] = "$NetBSD: s_cbrtf.c,v 1.4 1995/05/10 20:46:51 jtc Exp $";
  18 #endif
  19
  20 #include "math.h"
  21 #include "math_private.h"
  22
  23 /* cbrtf(x)
  24  * Return cube root of x
  25  */
  26 #ifdef __STDC__
  27 static const unsigned
  28 #else
  29 static unsigned
  30 #endif
  31         B1 = 709958130, /* B1 = (84+2/3-0.03306235651)*2**23 */
  32         B2 = 642849266; /* B2 = (76+2/3-0.03306235651)*2**23 */
  33
  34 #ifdef __STDC__
  35 static const float
  36 #else
  37 static float
  38 #endif
  39 C =  5.4285717010e-01, /* 19/35     = 0x3f0af8b0 */
  40 D = -7.0530611277e-01, /* -864/1225 = 0xbf348ef1 */
  41 E =  1.4142856598e+00, /* 99/70     = 0x3fb50750 */
  42 F =  1.6071428061e+00, /* 45/28     = 0x3fcdb6db */
  43 G =  3.5714286566e-01; /* 5/14      = 0x3eb6db6e */
  44
  45 #ifdef __STDC__
  46         float __cbrtf(float x)
  47 #else
  48         float __cbrtf(x)
  49         float x;
  50 #endif
  51 {
  52         float r,s,t;
  53         int32_t hx;
  54         u_int32_t sign;
  55         u_int32_t high;
  56
  57         GET_FLOAT_WORD(hx,x);
  58         sign=hx&0x80000000;             /* sign= sign(x) */
  59         hx  ^=sign;
  60         if(hx>=0x7f800000) return(x+x); /* cbrt(NaN,INF) is itself */
  61         if(hx==0)
  62             return(x);          /* cbrt(0) is itself */
  63
  64         SET_FLOAT_WORD(x,hx);   /* x <- |x| */
  65     /* rough cbrt to 5 bits */
  66         if(hx<0x00800000)               /* subnormal number */
  67           {SET_FLOAT_WORD(t,0x4b800000); /* set t= 2**24 */
  68            t*=x; GET_FLOAT_WORD(high,t); SET_FLOAT_WORD(t,high/3+B2);
  69           }
  70         else
  71           SET_FLOAT_WORD(t,hx/3+B1);
  72
  73
  74     /* new cbrt to 23 bits */
  75         r=t*t/x;
  76         s=C+r*t;
  77         t*=G+F/(s+E+D/s);
  78
  79     /* retore the sign bit */
  80         GET_FLOAT_WORD(high,t);
  81         SET_FLOAT_WORD(t,high|sign);
  82         return(t);
  83 }
  84 weak_alias (__cbrtf, cbrtf)